Understanding the physiological mechanisms underlying ventricular fibrillation (VF) has progressed with the application of nonlinear dynamics theory and voltage-sensitive optical techniques to the study of cardiac electrical wave propagation. The information gained about ventricular fibrillation during overt autonomic receptor activation has the potential to ultimately enhance the technological development of devices that could advance cardiac nursing practices and improve patient quality of life outcomes. The purpose of this investigation is to assess the nonlinear characteristics of VF in the isolated rabbit heart under pharmacologic conditions simulating overt autonomic receptor activation. ECG and voltage-sensitive image data will be collected under three different conditions: sympathetic receptor activation, parasympathetic receptor activation and no autonomic receptor activation. Before, during and after VF, both types of data will undergo recurrence quantification analysis (RQA), a nonlinear signal analysis technique. The specific aims are to: Determine if with exogenous activation of sympathetic adrenergic receptors with norepinephrine, and/or exogenous activation of parasympathetic muscarinic receptors with acetylcholine, is associated with changes in the RQA variables via analysis of ECG and voltage-sensitive image data before, during and after VF.